Effect of phase composition of the oxidic precursor on the HDS activity of the sulfided molybdates of Fe(II), Co(II), and Ni(II)

The catalytic HDS activities of unsupported sulfided molybdates of Fe(II), Co(II), and Ni(II) have been examined measuring the conversion of thiophene at 400°C under atmospheric pressure. The oxidic precursors employed included the hydrates AMoO₄·H₂O and the α- and β-AMoO₄ polymorphs (A = Fe, Co, or Ni). The previous finding that sulfided β-NiMoO₄ is a better HDS catalyst than α-NiMoO₄ is now generalized to the other two molybdate systems, suggesting that the tetrahedral environment of Mo in the β-isomorphs provides a more active A-Mo-S phase than the octahedral one in the α-molybdates. The reduced (nonpresulfided) molybdate samples showed lower HDS activities than those sulfided in pure H₂S. Prereduction followed by sulfiding seems to be an optimum procedure for the highest HDS activity of the nickel molybdates and also of β-FeMoO₄. It was found that NiMoO₄·H₂O renders a more active sulfided catalyst than the β-phase synthesized by calcination at 760°C, and this seems to be related to the in situ generation of β-NiMoO₄ with higher surface area during the activation of the hydrate at 400°C. The hydrated phase of cobalt behaved similarly, but that of iron, on the contrary, was a poorer catalytic precursor than β-FeMoO₄. It is suggested that decomposition of the hydrated compound in the case of Fe could generate a more crystalline material upon sulfiding due to the possibility of changes in the oxidation state of the metals (i.e., Fe²⁺ ⇄ Fe³⁺, Mo⁶⁺ ⇄ Mo⁵⁺) during thermal transformations in presence of evolved water.